Among active oxygen species, hydroxyl radicals have very strong oxidizing properties and can react with a very wide variety of organic compounds (contaminants). Therefore, hydroxyl radical-containing water is known to be useful as a cleaning liquid for decomposing or removing TOC in ultrapure water or decomposing or removing organic compounds deposited on the surface of a solid such as a wafer.
For example, Patent Document 1 discloses that hydroxyl radicals are produced in a cleaning method characterized by bringing an object to be cleaned into contact with ozone bubbles-containing water in the presence of a reducing substance.
Also for example, Patent Document 2 discloses that hydroxyl radicals are produced in a cleaning method characterized by including immersing an object to be cleaned in a liquid in which ozone is dissolved or bubbled, and applying ultraviolet light to the surface of the immersed object to clean the surface.
Conventionally, there has also been an ozone water supply method including transferring ozone water to the point of use in the presence of an ozone decomposition inhibitor, reducing the ozone concentration to a predetermined value at a place in the vicinity of the point of use by concentration control means, and then supplying the ozone water (see for example Patent Document 3). The concentration control means is means for decomposing dissolved ozone, examples of which include ultrasonic irradiation, ultraviolet irradiation, turbulence formation, agitation, heating, alkali addition, and hydrogen peroxide addition.